1. Field of the Invention
This invention relates to a packet transfer device, a semiconductor device and a packet transfer system, and more particularly to a packet transfer device for transferring packets between LANs, public servers and WANs, a semiconductor device included in the packet transfer device, and a packet transfer system.
2. Description of the Related Art
Recently, Internet services providing constant connections to the Internet, such as ISDN (Integrated Service Digital Network), ADSL (Asymmetric Digital Subscriber Line), and FTTH (Fiber To The Home), have become available to home users and SOHO (Small Office Home Office) users. Accordingly, there is an increasing demand of users for installing a public server in such an environment constantly connected to the Internet, for making various kinds of information open to the public.
Further, there is conventionally known a method in which a DMZ (DeMilitarized Zone) as a segment isolated by firewalls is provided for protection of a public server against unauthenticated access or malicious attacks from the Internet, and the public server is installed in the DMZ.
FIG. 36 shows an example of a conventional method of constructing a DMZ.
In FIG. 36, a public server 22 is connected to a LAN (Local Area Network) 20 via a firewall 21, and at the same time connected to a WAN (Wide Area Network) 24 via a firewall 23.
According to this construction, unauthenticated access to the public server 22 from the WAN 24 can be blocked by the firewall 23, while unauthenticated access to the public server 22 from the LAN 20 and indirect unauthenticated access to the LAN 20 performed by taking over the public server 22 can be blocked by the firewall 21.
Therefore, by using the DMZ, it is possible to prevent not only unauthenticated access to the public server 22, but also unauthenticated access to the LAN 20 from the WAN 24, which is attempted by taking over the public server 22.
However, the construction of the DMZ mentioned above requires expensive and sophisticated firewalls, which makes it difficult to introduce this method e.g. into average homes or SOHOs, both in terms of costs and techniques.